Aromatic vinyl, alkyl methacrylate or vinylcyanide resins are used in various applications owing to their transparency. However, they typically do not have sufficient strength to meet market demand. In order to overcome this problem, various attempts have been made to introduce rubber components into the resins. For example, there is a method in which a graft copolymer having a core-shell structure prepared by emulsion polymerization is added into the resins as the rubber component. According to this method, rubber properties or morphology (e.g., rubber content or particle size) can be controlled relatively easily, such that there is a relatively large capability for designing the balance of impact resistance and transparency. However, in providing sufficient impact resistance to a resin having a high aromatic vinyl monomer content, the required particle diameter of the rubber component is relatively large with respect to the size range generally achieved by emulsion polymerization.
Accordingly, various methods for growing rubber particles have been investigated. For example, there is a method for adding a core-shell graft copolymer to a methyl methacrylate-styrene copolymer resin, whereby the core-shell graft copolymer is prepared by graft-polymerizing a monomer with rubber particles grown by agglomerating a small-particle resin component to have a particle diameter of 0.5 μm using an electrolyte such as acid or salt (e.g., see Japanese Examined Patent Application Publication No. 63-47745). This agglomeration method, which uses a water-soluble electrolyte, suffers from increased amounts of scale produced during polymerization due to the instability of the latex caused by addition of the electrolyte. Moreover, the method has a low upper limit of the particle size of the agglomerated rubber particles. Thus, making a large-particle graft copolymer according to this method is difficult.
In other methods, a graft copolymer is added to an aromatic vinyl-acryl copolymer resin, wherein the graft copolymer is prepared by graft-polymerizing a monomer with rubber particles grown using an acid-group-group-containing copolymer having a predetermined composition (e.g., Japanese Unexamined Patent Application Publication Nos. 60-118734 and 2002-20566). According to these methods, agglomerated rubber particles can be easily produced relative to those obtained by emulsion polymerization methods, and good impact resistance can be achieved. However, the transparency is not high enough to allow the resin to be applied for uses requiring high transparency. As noted above, conventional core-shell graft copolymer resin particles have failed to achieve high impact resistance without impairing the good transparency of the aromatic vinyl, alkyl methacrylate, or vinylcyanide resins.
Thus, the performance of core-shell graft copolymer resin particles needs to be improved.